otelhttp: export Event type for WithMessageEvents

[sonalgaud12]

This PR fixes issue #8132

The WithMessageEvents option previously accepted an unexported type (event).
This change exports the type as Event.

[sonalgaud12]

Hello @dmathieu , OpenTelemetry Go enums usually prefix constants with the type name (e.g. SpanKindServer, SpanKindClient). So shall I change ReadEvents, WriteEvents to EventRead, EventWrite .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.8%. Comparing base (7ff03d6) to head (1bd97b2).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8153   +/-   ##
=====================================
  Coverage   81.8%   81.8%           
=====================================
  Files        194     194           
  Lines      13362   13377   +15     
=====================================
+ Hits       10936   10952   +16     
+ Misses      2029    2027    -2     
- Partials     397     398    +1     

Files with missing lines	Coverage Δ
instrumentation/net/http/otelhttp/config.go	83.0% <ø> (+0.9%)	⬆️

... and 7 files with indirect coverage changes

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[flc1125]

@sonalgaud12 Please fix the lint; we can use make ci for local verification.

[dmathieu]

Nit:

Suggested change

	unspecifiedEvent Event = iota
	unspecifiedEvents Event = iota

[flc1125]

In fact, the modified expected and actual positions are incorrect; the pre-modified ones are accurate.

Of course, we could do it this way: assert.Equal(t, 1, int(otelhttp.ReadEvents), "ReadEvents should be 1")

It is worth noting that we still lack validation for unspecifiedEvents.

[flc1125]

Only remaining issues